Subject: How are allele probabilites calculated?

In the Egyptian line there were about 5 generations in a row of sibling 
marriage and offspring.  What is the probability that the offspring of the 5th 
sibling generation would share an allele with someone from the first sibling 
generation?  Einstein married his cousin and his parents were also cousins.  
What is the allele probability between Einstein's kids and his parents?  With 
continual inbreeding will the number of possible alleles decrease?  Increasing 
the likelihood that related people in the same line would share alleles?

Re: How are allele probabilites calculated?